High power and high intensity laser systems are very desirable. However, such high power and high intensity lasers are very hard to obtain with a high quality beam and a short pulse duration.
Laser amplifiers take an input laser beam from an external laser oscillator and amplify the input laser beam. Increasingly more intense and more powerful laser beams are achieved by increasing amplification power. However, conventional laser amplifiers have design and performance aspects that limit and even reduce achievable power and intensity gains. At high power and high intensity, heat generated by the laser pump light can create thermal optical effects and thermal stresses in laser and amplifying systems which distort the light beam, making conventional laser and amplifying systems inefficient or even inoperable. Furthermore, the energy contained in high power and high intensity laser beams can permanently damage, if not instantly vaporize, components of conventional laser and amplifying systems.
A limit on high power and high intensity laser amplification is the B-integral effect. The B-integral effect describes the relationship between the refractive index of a material and the intensity of illumination. Thus, a light beam with a non-uniform intensity distribution, such as a Gaussian intensity profile, has higher indices of refraction in areas of higher light intensity. Varying illumination intensities and thus varying indices of refraction also occur due to non-uniform energy densities resulting from laser pumping sources. The refractive index of the material determines the phase velocity of light through it, and thus the effective optical path length. As a result, phase delays occur in the regions of higher intensity, distort the focus of the light beam and limit the gains in intensity and power. A varying index of refraction also alters the optical path of affected portions of the beam, causing the whole beam or portions of the beam to collapse into focus points. The B-integral effect becomes more pronounced under high power and high intensity amplification because of the greater variances in illumination levels.
As a result of the B-integral effects and other sources of distortion to the light beam (such as optical imperfections in the laser path), high power and high intensity amplification in conventional laser amplifiers creates regions of heat accumulation (i.e., hot spots). Hot spots occur in areas of imperfections that disrupt the laser, dissipating energy into the surrounding regions. Hot spots may also form as a result of non-uniform pumping that causes varying levels of heat (e.g., heat gradients) to develop in different regions within the amplifier. As a region heats, it further distorts the refractive index profile in the laser amplifier, leading to still greater heat accumulation. This cycle of increasing heat and distortion continues until either the laser amplifier breaks down or destructive optical interference due to, for example, phase delays, prevents further gains in intensity and power.
For at least the above reasons, conventional laser amplifier designs are prone to hot spot formation and are limited in achievable gains in intensity and power. Hot spot formation also enhances inefficiencies in conventional laser amplifiers since much of the amplifying laser light energy is lost as it is converted into waste heat. Furthermore, conventional laser amplifiers are not well designed to withstand hot spots and rapidly break down under high power and high intensity amplification, requiring expensive repair and replacement of parts.
To manage these high temperatures, a means of active heat removal is generally advantageous. Conventionally, the non-optical surfaces of the laser crystal rod are cooled by the forced convection of a fluid, which is usually water. Alternatively, these surfaces can be thermally connected to a heat sink of sufficient mass to absorb the waste heat. However, due to the geometry of the active laser volume and the relatively low thermal conductivity of the laser crystal rod, high temperatures and large temperature gradients may persist.
Accordingly, there is a need for systems and methods for amplifying light that effectively produces high power and high intensity laser beams, but minimizes the formation of harmful hot spots and/or is robust enough to withstand the hot spots that do develop.